Static discharge is a naturally occurring electronic phenomenon. Triboelectric charge, often referred to as static electricity, builds up in all materials to some degree, and is eventually discharged as the charge traverses a path toward an electrical ground. Static electricity buildup is strongest in insulative materials.
Circuit boards contain microcircuitry which can be easily damaged by electrostatic discharge of relatively small magnitude. Magnitudes as small as 50 volts can permanently damage these devices. For comparative purposes, to illustrate the extreme sensitivity of microcircuits to this phenomenon, a visible charge from a human hand to a door knob in winter will often exceed 10,000 volts. Thus, extreme caution must be taken in protecting such microcircuit components from electrostatic discharge.
To provide protection from electrostatic discharge for packaged articles, a principle of physics referred to as the Faraday cage effect is employed. Electricity does not penetrate a conductive enclosure. The static electric charge will go around the enclosed space, seeking the path of least electrical resistance en route to ground. By surrounding a static sensitive article with a conductive enclosure, the article is shielded from electrostatic discharge originating outside of the enclosure.
Packaging is considered to be in the conductive range if it has a measurable surface resistivity of less than 10.sup.5 ohms per square inch. U.S. Pat. No. 4,610,353 to Young discloses a three-ply corrugated board used to form a container wherein each ply of the board has sufficient carbon embedded therein to make that ply electrically conductive, thus providing a Faraday cage to shield a packaged article from electrostatic discharge.
However, the use of such a conductive enclosure for packaging an electrostatic sensitive article presents other disadvantages. Conductive materials of this type can slough to create a dust, or particulate, of conductive material. Some amount of sloughing or spalling of conductive dust will inevitably occur, but the incidence of its occurrence is greatly increased if the article to be packaged contacts a wall of the cell as it is placed therein, as is often the case when packaging a microcircuit component. When a conductive particle contacts a microelectronic component, the particle can form an undesired short circuit or a misdirected pathway, thus shunting the electrical charge and resulting in temporary or permanent damage. Therefore, while it is desirable to provide a conductive enclosure to shield microcircuits from electrostatic discharge, it is also necessary that the packaged article be shielded from sloughing of particulate conductive material.
Attempts have been made to limit the effects of conductive spall. U.S. Pat. No. 4,482,048 to Blodgett discloses a divider used in packaging which is made from corrugated board having a conductive layer of material laminated between two outer webs. The inner conductive layer shields the packaged article from electrostatic discharge while the outer corrugated board webs prevent conductive particulate matter from spalling and contacting electrical circuitry.
Although the web residing nearest to the packaged article of the Blodgett patent effectively blocks the packaged article from conductive spall which originates from the interior conductive layer, electrostatic discharge protection is compromised with this structure because this corrugated board web is presumably a paperboard web and paperboard is an insulator. One of the best generators of static electricity is movement of an electrical conductor, such as an electrical circuit of a circuit board, over an insulator, such as a paperboard web. The disposition of an insulative layer between the conductive layer and the packaged article of the Blodgett patent therefore creates a static electricity generator in direct contact with the electrical circuit board or compound which the Blodgett patent is attempting to protect from undesirable contact with static electricity.
There are other inherent disadvantages associated with the use of corrugated board for packaging articles in the manner of the Blodgett patent. Although the fluted layer common to all corrugated board provides rigidity to physically protect the packaged article, the physical protection afforded can be far greater than what is actually needed, especially in light of the fact that the fluted layer creates a void or empty space which is not used. These voids take up shipping volume, resulting in increased handling expenses. Due to the flutes, corrugated board is also difficult to cut cleanly. The knife tends to direct the flutes in an undesired direction. Additionally, the outer webs of corrugated board tend to be abrasive, which can result in physical harm to the article during packaging. Lastly, corrugated board is typically Kraft paper colored, thus conveying the presumption that it is disposable, regardless of whether or not its useful life has expired. Inadvertent disposal of more expensive reusable packaging material is wasteful and can drive up the expense of packaging.
Anti-static, or dissipative, materials are defined as materials which do not generate a triboelectric charge. Anti-static materials generally have a surface resistivity of from 10.sup.9 to 10.sup.14 ohms per square inch. Anti-static materials might be considered static neutral. While an anti-static enclosure will not cause electrostatic discharge (ESD), an anti-static enclosure does not, by itself, provide static shielding either.
Because anti-static materials do not conduct a static charge, they can be used for cushioning or wrapping static sensitive articles before they are placed within conductive enclosures, thus providing protection against conductive spall. U.S. Pat. No. 4,658,958 to McNulty discloses a bag having two plies of anti-static (polyethylene) material and an electrically conductive fabric or mat embedded therebetween to provide a shield from electrostatic discharge for a bagged article.
Although the McNulty bag may prove effective in protecting a bagged article from electrostatic discharge, it does not, by itself, provide the adequate rigidity for physical protection of a microcircuit component. In order to provide physical protection, the bagged article must be placed within another cell in a container. This results in additional material and material handling costs, requiring a bag enclosure, additional labor costs associated with bagging and additional shipping costs due to reduced packaging density. Reduced packaging density is due to the fact that each cell must hold an article within a bag, rather than just the article itself. Thus, for a container having a given volume, use of bag packaging will necessitate larger sized cells resulting in the packaging of fewer articles per container.
In addition to providing electrostatic and physical protection, a container for packaging microcircuit components should also provide protection from chemical corrosion. Due to availability and relative cost, paper products are often used to form packaging containers for shipping articles. In the paper making process, caustic chemicals such as sulfur are added to the slurry in order to split out or soften wood fibers. Subsequently, after a container has been formed from the paper, sulfur dioxide and other corrosive gases trapped within the paper can escape to react with water in the air, eventually condensing upon the packaged article with a slight change in temperature. This condensation can corrode the article. In effect, a very minor scale acid rain takes place within the paper container. In order to chemically protect a microcircuit component, the sulfur must either be physically blocked or chemically bonded to another substance.
It is therefore an object of this invention to provide a conductive enclosure which adequately shields electrostatic sensitive articles from static electric discharge.
It is another object of the invention to provide an anti-static packaging enclosure which effectively prevents the adverse effects of conductive spall.
It is a still further object of this invention to provide a rigid enclosure for physically protecting an electrostatic sensitive article, which at the same time both shields the packaged article from static electricity and prevents the adverse effects of conductive spall.
It is still another object of the invention to provide a cost effective container which effectively protects a packaged article from electrostatic, physical and chemical damage.